This invention relates to devices which use ion beams, specifically devices which use ion beams to treat a workpiece such as a substrate, for example, by implanting materials in the substrate or sputtering or depositing material on the substrate.
Ion beams are used in a variety of technologies to treat substrates. For example, ion implantation is a process of generating an ion beam, focusing the beam, analyzing the beam to separate a species of ions for implantation and directing it towards a substrate to implant the ions in the substrate.
A positive ion beam does not have an equal density of ions and electrons and has an inherent potential that is typically distributed nonuniformly across a cross-section of the beam. (This inherent potential of a beam is also known as the space charge of the beam.) Because the ions in the beam repel each other, they result in the beam diverging and losing density and focus as the beam travels along the beam path. This tendency depends on the perveance of the beam. A beam that has a high current and a low energy (e.g., less than 15 KeV preferably less than 5 KeV), and therefore a low speed, typically has a high perveance.
In the case of devices that use ions beams to treat substrates, fast moving particles collide with residual gases in the device and the walls of the device and generate low energy ions and secondary electrons. A positively charged ion beam traps these electrons and simultaneously rejects the positive ions. However, these electrons are typically not present in sufficient density to completely compensate for the space charge of the beam (typically referred to as "neutralizing" the space charge of the beam). This is especially the case for low energy beams, which have a low cross-section for producing secondary electrons.
Various methods have been used to further neutralize the space charge of ion beams. The goal of these methods is to generally increase the current and the stability of an ion beam by reducing the space charge of that beam. Examples of these methods include generating free electrons for introduction into the path of the beam for neutralizing the beam potential. The electron may be generated by electron sources or by plasma sources which generate a plasma formed mainly of positive ions and free electrons.